1. Field of the Invention
The present invention relates to a capacitive input device used as, for example, a pointing device of a personal computer.
2. Description of the Related Art
In recent years, capacitive input devices have been generally used for various electric apparatuses called portable terminals, such as mobile phones, cameras, and PDAs (Personal Digital Assistants).
As an example of the input devices, an input device has been known in which a counter electrode having a predetermined area adheres to an inner surface of a casing of an electric apparatus. In such an input device, the counter electrode is formed of a sheet-shaped conductive metal plate, such as a thin copper plate, and is provided along the inner surface of the casing so as to have a little large area. Since the counter electrode is formed of a very thin film, it is possible for the counter electrode to adhere to the inner surface of the casing so as to correspond to the curved surface of the casing. When a conductor, such as a finger, approaches and comes into contact with an outer surface of the casing opposite to the counter electrode, capacitance is formed between the conductor and the counter electrode. The capacitance varies according to the area and distance where the counter electrode and the conductor are opposite to each other.
The casing is made of a material having a high dielectric constant other than a metallic material, that is, synthetic resin, such as plastic.
Further, the input device comprises detecting means for detecting the capacitance, clock signal generating means, signal delay means, delayed signal detecting means, and A/D converting means.
The clock signal generating means continuously outputs a regular pulse signal having a predetermined frequency. The signal delay means comprises a variable capacitance portion and a resistor R connected between the variable capacitance portion and the clock signal generating means. In addition, the delayed signal detecting means includes an AND circuit and smoothing means comprising a resistor and a capacitor that are connected to the rear stage of the AND circuit. A clock signal that is output from the clock signal generating means and does not pass through the signal delay means and another clock signal that is output from the clock signal generating means and passes through the signal delay means are input to input portions of the AND circuit, respectively. Then, the output of the AND circuit is input to the smoothing means. The A/D converting means detects an output voltage from the smoothing means at a predetermined sampling period and converts it into a digital signal. Then, the A/D converting means outputs the digital signal to a control unit composed of a CPU. Then, the control unit obtains various information items from the digital signal output from the A/D converting means, and allows an electric apparatus to perform a predetermined operation based on the information items (for example, see Japanese Patent Application No. 2002-306887).
However, in the conventional capacitive input device having the above-mentioned structure, in the case in which the counter electrode adheres to the inner surface of the casing of the electric apparatus, if it is difficult for the counter electrode to adhere to the inner surface due to the positional relationship therebetween, the counter electrode must adhere to an inner case of the electric apparatus. In this case, a space, that is, an air layer may be formed between the surface of the counter electrode and the casing of the electric apparatus. When a space is formed between the surface of the counter electrode and the casing of the electric apparatus, it is difficult to adjust the sensitivity of the counter electrode, resulting in lowering the operability of the input device.
Further, a recent trend in various electric apparatuses is to provide casings having a metallic tone. Therefore, when the surface of the casing, which is an operating surface, is made of a metallic material or is plated with a metallic material, the erroneous operation of the counter electrode may occur due to the conductivity of the metallic material. Thus, the surface of the casing must be made of a non-conductive material, such as resin, which results in deteriorating the appearance of the electric apparatus.